How To Know What Size Your Tires Are | Read The Sidewall

Your tire size is stamped on the sidewall in a code like P225/65R17, and the driver’s door placard should match it.

If you’re wondering how to know what size your tires are, you don’t need a tape measure. The answer is already on the car. Start with the sidewall of the tire that’s on the vehicle now, then check the sticker on the driver’s door jamb. That second check matters because a used car may be riding on the wrong tires.

The size code looks cryptic at first glance, but it breaks into plain parts: width, sidewall height ratio, construction type, wheel diameter, load index, and speed rating. Once you know where each piece sits, you can read a tire in seconds and buy a matching replacement with far less guesswork.

Where To Find The Number

You have three solid places to check, and each one tells you something a little different.

• Tire sidewall: Best for reading the full code already mounted on the car.
• Driver’s door placard: Best for the factory-recommended size and cold pressure.
• Owner’s manual: Handy when the placard is faded or you want trim-specific fitment notes.

The sidewall is the fastest starting point. Run your hand along the outer side of the tire and look for a string like P225/65R17 102H. On some tires the letters and numbers sit close to other markings, so wipe off dust first and scan slowly.

The door placard is the better tie-breaker. It usually sits on the driver’s door edge, the jamb, or the B-pillar. The NHTSA TireWise pages point drivers to the placard and owner’s manual when checking tire labeling and replacement details.

Reading Tire Size Numbers On The Sidewall

Let’s use P225/65R17 102H. That single line tells you far more than the width of the tread. It also tells you the shape of the sidewall, the wheel size it fits, and the load and speed class the tire was built to carry.

Breaking The Code Into Plain English

P marks a passenger tire. Some vehicles use LT for light truck tires, and temporary spares can start with T. Next comes 225, the tire width in millimeters from sidewall to sidewall. Then 65 tells you the sidewall height as a percentage of width, not a direct inch or millimeter figure.

R means radial construction, which is what most modern road tires use. 17 is the wheel diameter in inches, so this tire fits a 17-inch wheel. The last pair, 102H, is the service description: the load index and speed rating.

That last part trips up a lot of shoppers. Two tires can share the same width and wheel diameter but still differ in load index or speed rating. If you buy by the first four characters alone, you can still end up with the wrong tire.

When Load And Speed Matter Most

Pay close attention to that service description on heavier SUVs, pickups, vans, and sporty cars. Those vehicles put more stress on the tire, so the last two markings are not filler. They are part of the fitment, just like the wheel diameter.

What Else Sits Near The Size Code

You’ll also see the DOT code, max load, max pressure, treadwear grade, and brand or model line. Those markings help with age, recall checks, and tire selection, but they are not a substitute for the vehicle placard. Michelin’s tire markings explainer also notes that the max pressure on the sidewall is not the same thing as the car’s recommended operating pressure.

Marking	What It Means	Why You Should Care
P	Passenger tire	Tells you the tire type the size code starts with
225	Section width in millimeters	Affects fit, contact patch, and clearance
65	Aspect ratio	Shows sidewall height relative to width
R	Radial construction	Matches the tire build used on most cars
17	Wheel diameter in inches	Must match the wheel exactly
102	Load index	Shows how much weight one tire can carry
H	Speed rating	Sets the tire’s approved speed class
DOT code	Plant and build date data	Useful for age checks and recall searches

How To Know What Size Your Tires Are When Numbers Don’t Match

This is where many people get stuck. The tire on the car says one thing. The door sticker says another. Or the front and rear tires are different sizes. None of that means you have a problem right away, but it does mean you need to sort out what the car was meant to wear.

Start With The Placard, Not The Tire Shop Listing

The placard is your cleanest source because it came with the vehicle. It lists the recommended tire size and the cold inflation pressure for that setup. If the current tire does not match it, treat the placard as your baseline unless the car has a documented alternate factory fitment package.

Some trims ship with staggered setups, where the rear tires are wider than the fronts. Sports cars do this often. In that case, check whether the placard lists separate front and rear sizes, or whether the owner’s manual lists more than one approved combination.

Then Check These Trouble Spots

• Used-car swaps: A past owner may have fitted whatever was cheap or on hand.
• Wheel changes: An 18-inch aftermarket wheel means the tire size must change to keep rolling diameter close.
• Winter sets: A second wheel-and-tire package may be approved, but it still needs the right load and speed class.
• Temporary spare: It will not match the full-size tire code, and that’s normal.

If the placard is missing and the current tires look suspect, the owner’s manual is next. Dealer parts departments can also pull factory fitment by VIN, which helps when a car had multiple wheel options from the start.

Common Tire Size Mistakes That Cost Money

The easiest mistake is buying by wheel diameter alone. A 17-inch wheel can wear many tire sizes, and the wrong one can rub the fender, throw off the speedometer, or leave the car with a harsh ride. Width and aspect ratio are just as tied to fit as the wheel diameter.

Another slip is ignoring load index and speed rating. That matters on heavier crossovers, trucks, and performance cars. A cheaper tire with the wrong service description may fit the wheel but still miss the vehicle’s spec.

One more trap: reading max pressure on the sidewall and inflating to that number. That figure belongs to the tire’s upper limit under its stated load, not the daily pressure your car calls for. The placard is the number to follow for routine inflation.

Check Point	What To Read	What A Good Match Looks Like
Sidewall	Full size code	Matches the placard or an approved factory alternate
Door placard	Recommended size and cold pressure	Used as the baseline for replacement shopping
Owner’s manual	Trim and wheel-package notes	Confirms approved front/rear combinations
Service description	Load index and speed rating	Meets or exceeds the vehicle requirement
Spare	Temporary-use marking	Not treated as the normal tire size for all four corners

A Five-Minute Check Before You Buy

If you want a clean, low-stress way to shop, use this order:

1. Read the full code on one front tire and one rear tire.
2. Photograph the door placard.
3. Match width, aspect ratio, construction, and wheel diameter.
4. Match the load index and speed rating, or go higher if the vehicle maker allows it.
5. Check whether the car uses the same size at all four corners.

That short routine saves you from the two most common headaches: ordering a tire that fits the wheel but not the car, or buying four matching tires for a vehicle that was built for a staggered setup. A quick photo of the placard also helps if you’re shopping online and want to compare listings later.

Once you’ve done that, the tire size code stops feeling like gibberish. It becomes a compact label that tells you what the car needs and what a replacement tire must match. That’s the whole trick: read the sidewall, verify it against the placard, and buy the full spec instead of one number from the code.